Anticholinergic drugs, such as the muscarinic acetylcholine receptor (mAChR) antagonist trihexyphenidyl, were the first accepted treatment for Parkinson's disease (PD) and are still in clinic use for this disorder and are among the most effective drugs available for treatment of dystonia. However, clinical utility of these compounds is limited by the severe central and peripheral adverse effects that are likely mediated by mAChR subtypes that are not related to the treatment of these disorders. In addition, the site of action of anticholinergics in the treatment of PD and dystonia is largely unknown. It is commonly believed that both PD and dystonia are circuit disorders involving the basal ganglia (BG) dysfunction and also considered as hypercholinergic disorders. Evidence suggests that M1 and M4 are the most abundant mAChR subtypes expressed in principal projection neurons in the striatum, a major input structure in the BG. Hyperactivity of striatal projection neurons (also termed medium spiny neurons, MSNs) is postulated to be associated with motor deficits in PD and dystonia and increased cholinergic signaling in the striatum has also been implicated in PD and certain forms of dystonia. Based on net excitation of MSNs by mAChR activation, drugs that selectively block mAChR subtypes that mediate the net excitation of striatal MSNs, but devoid of activity for other subtypes, might be expected to have therapeutic effects on PD and dystonia without undesired adverse effects. However, due to lack of highly selective mAChR ligands, definitive determination of the individual mAChR subtypes involved in physiological and pathophysiological functions in the striatum has not been possible until recently. Now we have developed a series of novel compounds that display unprecedented selectivity for either M1 or M4 subtype with no detectable activity at any other mAChR subtypes. In this proposal, we will take advantage of these novel, highly selective mAChR ligands and transgenic mice in which gene coding a specific mAChR subtype is deleted, to definitively determine the roles of M1 and M4 in regulating physiological functions of striatal MSNs. Specifically, we will rigorously test the hypothesis that 1) M1 and M4 mACh are involved in modulation of neuronal excitability in MSNs; 2) M1 mAChR activation potentiates NMDA receptor currents in MSNs; 3) M1 and M4 mAChR play important roles in modulation of transmission and long-term plasticity at corticostriatal synapses in MSNs. Finally we will determine the ability of selective M1 antagonist and M4 potentiator to alleviate motor deficits of rodent models of PD and dystonia. The results of these studies will provide critical new information regarding the different roles of M1 and M4 mAChR subtypes in physiological and pathophysiological functions in the striatum and provide the basis for the development of improved anticholinergic therapies for PD, dystonia and other movement disorders that could be devoid of the severe adverse effects.